Quartz glass cylinder for production of an optical component and method for production thereof

ABSTRACT

The aim of the invention is to improve a known quartz glass cylinder for the production of an optical component, comprising an inner drilling, which is mechanically machined to size and provided with an etched structure by means of an etching treatment, subsequent to the mechanical machining, such that in the application thereof for production of pre-forms and optical fibers, few bubbles arise along the boundary surface between core and sleeve. Said aim is achieved, whereby the etched structure comprises striations with a maximum depth of 2.0 mm and a maximum width of 100 μm. A method for production of such a quartz glass cylinder mechanically machined to size is characterized in that the mechanical machining comprises several serial removal processes with successively lower removal depths, whereby after the last removal process the inner drilling has sub-surface striations with a maximum depth of 2 mm and the inner drilling is subsequently subjected to an etching treatment such that an etching removal with a maximum depth of 50 μm is achieved.

The present invention relates to a quartz glass cylinder for producingan optical component with an inner bore which is mechanically treated toa final dimension and is provided with an etched structure due to anetching treatment following mechanical treatment.

Furthermore, the present invention relates to a method for producing aquartz glass cylinder by mechanically treating the inner bore of thequartz glass cylinder to a final dimension and by subsequentlysubjecting the same to an etching treatment.

Such quartz glass cylinders serve to produce optical fibers and preformsfor optical fibers. They are used as so-called “jacket tubes” tooverclad core rods with cladding glass. Overcladding can be carried outby collapsing and elongating a coaxial arrangement of the hollowcylinder of quartz glass in the inner core of which the core rod isinserted. Preforms are thereby produced, from which optical fibers aresubsequently drawn. It is also known that the hollow cylinder iscollapsed onto a core rod during fiber drawing, the last-mentionedmethod being called “ODD (overclad-during-drawing) method”.

A quartz glass cylinder and a method for producing the same according tothe above-mentioned type are known from DE 102 14 029 A1. In the methoddescribed therein, a tube of synthetic quartz glass is manufactured byproducing a soot body by flame hydrolysis of SiCl₄ and vitrifying saidsoot body to obtain a hollow cylinder of quartz glass and bysubsequently treating the quartz glass block by means of a core drill.For a precise finishing operation of the tube obtained in this way it issuggested that the inner wall thereof should be reworked by means of ahoning machine and should be honed in a final step using an abrasive ofthe fineness grade # 800. To reduce surface tensions and to eliminatedamage caused by surface treatment, the treated tube of quartz glass isetched in hydrofluoric acid.

In parallel therewith, a so-called core rod is produced which consistsof core glass of germanium-doped SiO₂ and which is surrounded by acladding glass of undoped SiO₂.

For producing an optical fiber the core glass rod is inserted into theinner bore of the hollow cylinder of quartz glass and fixed therein withformation of a coaxial assembly. Starting with its lower end, saidassembly is supplied from above to an electrically heated fiber drawingfurnace at a predetermined feed speed and is heated therein to atemperature around 2180° C. and softened zonewise in this process. Anoptical fiber having an outer diameter of 125 μm is drawn off from thesoftened region. Due to plastic deformation in the furnace, the annulargap between the core rod and the hollow cylinder of quartz glass isclosed, a negative pressure being maintained in the gap.

EP-A 598 349 describes a thick-walled quartz glass cylinder forproducing a large-volume preform for optical fibers. The thick-walledcylinder is collapsed onto a core rod during elongation. Said method isknown under the name “RIC (rod in cylinder) method”. Several proceduresare suggested for producing the quartz glass cylinder. The firstprocedure consists of two steps. In the first step of the procedure, acylindrical quartz glass blank is provided. In the second step the blankis mechanically drilled for forming a central bore either by using acore drill, or it is subjected to a hot upsetting method to produce abore. In the second procedure, porous silicic acid soot is deposited ona heat-resistant substrate tube, said tube is then removed, and the soottube obtained thereby is dehydrated and vitrified.

It has been found that the preforms produced according to the knownmethods often comprise bubbles on the boundary surface between core rodand hollow cylinder, and that the quality of the fibers drawn from suchpreforms is often also inadequate. Particular attention is here paid toelongated bubbles along the boundary surface between core and cladding.These may result in low fiber strength and may particularly causeproblems during splicing of the fibers.

It is the object of the present invention to provide a quartz glasscylinder which, when used for producing preforms and optical fibers,avoids the above-mentioned drawbacks. It is a further object of thepresent invention to indicate a method for producing such a quartz glasscylinder.

As for the quartz glass cylinder, said object, starting from theabove-mentioned quartz glass cylinder, is achieved according to theinvention in that the etched structure comprises cracks having a depthof not more than 2.0 mm and a width of not more than 100 μm.

It is possible by way of a mechanical treatment (particularly bydrilling, honing and grinding) using known honing and grinding methodsand commercially available devices suited therefor to produce a hollowcylinder of quartz glass having an outer diameter of more than 100 mmand a length of 2 m and more, said hollow cylinder being distinguishedby an exact cylinder symmetry with accurate circular cross-section and asmall dimensional deviation in the range of 1/100 mm.

So far it has been assumed that, apart from an exact dimensionalaccuracy and cylinder symmetry, the surface roughness of themechanically treated hollow cylinder constitutes a decisive qualitycriterion for the suitability of the cylinder to be used for cladding acore rod in an RIC method. This becomes, for instance, apparent from theabove-mentioned EP 0 598 349 A1, where the quality of the mechanicallytreated inner surface of the hollow cylinder of quartz glass is definedby way of roughness data.

However, it has been found that preforms and fibers obtained by usingquartz glass cylinders having a mechanically treated inner bore oftencomprise bubbles on the boundary surface towards the core rod material,i.e. even in cases where quartz glass cylinders with a very smooth andthoroughly treated inner surface have been used. A definite correlationbetween the roughness of the inner bore of the hollow cylinder and thequality of the resulting boundary surface in a preform obtainedaccording to the RIC method or the quality of the fiber drawn therefromcould not be detected. Problems arose especially during use ofparticularly thick-walled quartz glass cylinders with outer diameters ofmore than 100 mm.

Surface roughnesses are normally determined with the help of measuringmethods in which a needle of a surface roughness measurement devicetravels along a predetermined path over the surface to be measured,thereby recording a surface profile. Detailed studies have shown thatdue to the mechanical treatment of the hollow cylinder cracks(subsurface cracks) arise in the near-surface area, said cracks beingnormally closed and thus not detectable with the standard roughnessmeasurement methods.

It has now been found that the depth of such cracks may even besurprisingly large in cases where the damage layer produced by theprevious removal process has been successively decreased in subsequenttreatment stages and only small forces are still acting on the surfacein the last treatment steps, resulting in a small removal. Nevertheless,these cracks would probably be harmless because they are closed andwould therefore melt and completely disappear while the quartz glasscylinder is collapsed onto the core rod.

This, however, is no longer true if the hollow cylinder of quartz glassis subjected to the standard cleaning process in an etching solutiondirectly before its use. In this etching process, the existingsubsurface cracks are opened, i.e. over their whole depth, whilesimultaneously expanding in lateral direction during the etchingprocess. Only these cracks that have been broadened due to acid cleaningmay lead during the subsequent collapsing process to defects in the areaof the boundary surface between core rod and hollow cylinder of quartzglass if they can no longer be closed. And the problems are increasingthe broader and deeper the cracks in the etched structure are, thehigher the viscosity of the surface is during collapsing and the shorterthe collapsing period.

Since thick-walled quartz glass cylinders with outer diameters of morethan 100 mm normally show a lower viscosity than thin-walled cylindersduring collapsing in the area of their inner bore, the problemsaccompanying an etched structure that no longer fuses are increasinglyfound in thick-walled quartz glass cylinders. With larger gap widthsbetween hollow cylinders of quartz glass and core rod it is more likelythat the defects in the inner surface fuse before contact with the corerod than in the case of small gap widths. These manifold conditionsregarding the presence or absence of defects in preforms and fibers dueto mechanical treatment and etching of quartz glass cylinders areprobably the reason why this problem has so far not been recognized.

As has been mentioned, the disadvantageous impacts of the etchedstructure on the preform and fiber quality can be reduced by suitablehot processes, such as a very slow collapsing. However, an inner surfaceoptimized with respect to the prevention of surface defects is preferredin consideration of the costs incurred by hot processes.

Hence, it has become apparent that a decisive factor for thequalification of the cylinder for the RIC method is not primarily thesurface roughness, but the etched structure produced due to the etchingprocess by expansion of the existing near-surface cracks. Therefore, themain focus of the invention is not the surface roughness but, on the onehand, the minimization of subsurface cracks in the area of the innerbore of the quartz glass cylinder, which cracks are caused by themechanical treatment, and on the other hand the restriction of theexpansion of the cracks due to a final etching process to a maximumvalue, so that they can adequately melt also in the case of adverseconditions during the collapsing process (low temperature, rapidcollapsing process, small gap width) and defects on the boundary surfacebetween hollow cylinder of quartz glass and core rod are prevented orreduced.

The decisive criteria are the crack depth and the crack width in theetched structure after the etching process. After the mechanicaltreatment cracks deeper than 2.0 mm must not remain in the inner wall ofthe quartz glass cylinder, and after the etching process the resultingetched structure must not contain cracks broader than 100 μm at the sametime.

It already follows from the above explanations that a cylindermechanically treated to a final dimension is within the meaning of thisinvention a cylinder whose inner surface has been mechanically treatedto a final dimension and which is subsequently cleaned by etching.Uniform etching processes do not cause a change in the geometrical finalshape of the hollow cylinder (for instance a bend or ovality in crosssection).

A particularly high quality of the boundary surface between hollowcylinder and core rod is accomplished when the etched structurecomprises cracks having a depth of not more than 1.0 mm and a width ofnot more than 50 μm, preferably in an etched structure with crackshaving a depth of not more than 0.5 mm and a width of not more than 20μm.

On the other hand, particularly small and narrow cracks in the etchedstructure, whose elimination or prevention is extremely time-consumingand expensive, will no longer be noticed negatively even under adverseconditions during the collapsing process. Therefore, for reasons ofcosts it has turned out to be advantageous when it is not attempted toavoid or eliminate cracks altogether, but to allow an etched structurecomprising cracks with a depth of at least 30 μm and a width of at least5 μm.

Preferably, the quartz glass cylinder of the invention has an outerdiameter of at least 150 mm.

The above-described measures with respect to the etched structure can bepositively noticed particularly during use of large-volume hollowcylinders having outer diameters of at least 150 mm, for large-volumequartz glass cylinders are in general difficult to heat all over duringthe collapsing process and thus show in the area of their inner bore acomparatively high viscosity which intensifies the problems accompanyingan etched structure that does no longer fuse.

As for the method, the above-mentioned technical object, starting from amethod of the above-indicated type, is achieved according to theinvention in that the mechanical treatment of the quartz glass cylindercomprises a plurality of subsequent removal processes with asuccessively decreasing removal depth, the inner bore comprisingsubsurface cracks with a depth of not more than 2 mm after the lastremoval process, and that the inner bore is subsequently subjected to anetching treatment in such a way that an etching removal with a depth ofnot more than 50 μm is achieved.

The mechanical treatment of the surface of the inner bore automaticallycreates cracks. The crack depth is successively reduced by repeatedgrinding and polishing or honing steps to such an extent that the crackdepth is not more than 2 mm. The crack depth which can be toleratedaccording to the invention permits the use of hollow cylinders whichrequire a less troublesome mechanical treatment of their inner wall andwhich can therefore be produced at comparatively low costs.

After completion of the mechanical treatment of the quartz glasscylinder the surface of the inner bore thus comprises closed crackshaving a depth of not more than 2 mm. Due to the subsequent etchingprocess, said cracks are opened. The crack depth does not change in thisprocess, but the crack width. Said width is about twice as large as theetching removal in the area. With an etching removal of not more than 50μm in the area, this will thus yield an etched structure with crackshaving a maximum crack width of about 100 μm.

As for the advantageous effect of such an etched structure on thequality of the boundary surface between quartz glass cylinder and corerod after collapsing of the hollow cylinder in an RIC method, referenceis made to the above explanations regarding the quartz glass cylinder ofthe invention.

A particularly high quality of the boundary surface between hollowcylinder and core rod is accomplished when the etching treatment yieldsan etching removal with a depth of not more than 25 μm, preferably anetching removal with a depth of not more than 10 μm.

Such an etching removal in the area yields a maximum expansion of theexisting cracks in lateral direction of 50 μm and 20 μm, respectively.

Preferably, the etching treatment results in an etching removal with adepth of at least 2.5 μm.

The boundary quality will be further improved if the etching treatmentcomprises a first etching step in an etching solution containinghydrofluoric acid and a second etching step in an etching solutioncontaining nitric acid.

The first etching step in the etching solution containing hydrofluoricacid will effect a removal of the SiO₂ surface, so that the existingcracks are slightly expanded. The second etching step in an etchingsolution containing nitric acid will not effect a further removal of theSiO₂ surface, but the dissolution of existing contamination. Thepreceding crack expansion is conducive to the attack of the nitricacid-containing etching solution in the area of the cracks.

It has turned out to be advantageous to carry out the etching treatmentat a mean etching rate of not more than 3 μm/min.

A low etching rate of less than 3 μm/min helps to observe apredetermined etching removal, especially if said removal itself issmall. Preferably, the mean etching rate is not more than 1 μm/min,particularly preferably not more than 0.1 μm/min.

The quartz glass cylinder of the invention is preferably used forproducing a preform for an optical fiber in an RIC method by collapsingthe cylinder onto a core rod and by simultaneously elongating thecylinder with formation of the preform.

Equally preferred is a use of the quartz glass cylinder according to theinvention for producing an optical fiber in an RIC-ODD method bycollapsing the cylinder onto a core rod and by simultaneously elongatingthe same with formation of the fibers.

The invention will now be explained in more detail with reference to anembodiment and a patent drawing. In detail,

FIG. 1 is a schematic view showing a profile of a fire-polished glasssurface with initial cracks during progressive etching;

FIG. 2 is a photograph of the surface of a quartz glass cylinder of theinvention after mechanical treatment;

FIG. 3 is a photograph of the surface of the quartz glass cylinder ofFIG. 2 after an etching process in HF-containing etching solutionlasting for 1 min; and

FIG. 4 is a photograph of the same surface as in FIG. 3 after an etchingprocess in HF-containing etching solution lasting for 50 min.

The production of a quartz glass cylinder according to the OVD-methodwill first of all be described. To this end soot particles are depositedin layers by reciprocating a number of deposition burners on a carrierrotating about its longitudinal axis, with SiCl₄ being supplied to thedeposition burners and oxidized and hydrolyzed in a burner flame in thepresence of oxygen to obtain SiO₂. After completion of the depositionmethod and removal of the carrier, a soot tube is obtained that issubjected to a dehydration treatment and introduced in verticalorientation into a dehydration furnace and treated at a temperatureranging from 850° C. to about 1000° C. in a chlorine-containingatmosphere. The treatment lasts for about six hours.

The soot tube treated in this way is then vitrified in a vitrificationfurnace at a temperature in the range of about 1350° C. with formationof a tubular quartz glass blank consisting of synthetic quartz glass,whose outer wall is coarsely ground by means of an NC peripheralgrinder, which is equipped with a #80 grinding stone. The inner bore istreated by means of a honing machine, the degree of polish becomingcontinuously finer by exchanging the honing bars. The final treatment iscarried out with a #800 honing bar showing a removal of about 60 μm. Thephotograph of FIG. 2 shows the surface of the inner wall treated in thisway, which will be described further below in more detail.

The tube is subsequently etched in an etching solution containinghydrofluoric acid. In this etching solution an etching rate of about 1μm/min ensues at room temperature. The maximum surface roughness R_(max)in the area of the inner wall is thus 3.5 μm, and in the area of theouter wall it is 77 μm.

Due to the mechanical treatment of quartz glass surfaces by grinding orhoning, it is not only material that is removed, but subsurface cracksare also produced. Since such cracks are very narrow, there will be noperfect method for determining the same, i.e. neither surface roughnessmeasurements nor optical measuring methods are able to define suchsubsurface cracks quantitatively.

The only method, which is however not free from destruction, consists inmaking the near-surface cracks visible by etching the surface.Therefore, the depth of the existing subsurface cracks is determined ona piece of the tube in a separate test in that the tube piece is etchedin 68% hydrofluoric acid for such a long period of time that the crackbase can be detected optically or by means of a surface roughnessmeasuring device. The results of said crack depth measurements aresummarized in column 2 of Table 1.

FIG. 1 shows a schematic illustration of the changing shape of a crackin a fire-polished surface with an increasing etching duration, such anillustration being known from the literature. The illustrated profileschematically shows, at position “0” (etching duration=0 minutes), acrack of a specific depth which starts from the surface. After anetching duration of 2 minutes the crack has slightly expanded and hasformed a small crater at its end oriented towards the surface. The crackdepth, however, has not changed, starting from the new surface which isnow positioned slightly deeper. With an increasing etching duration of4, 8, 30 and 45 minutes, respectively, a considerably increasingexpansion of the crack can be observed without the depth thereofincreasing on account of the etching process. The lateral boundary wallsof the crack, however, are removed at about twice the speed as theplanar surface. Therefore, the crack width increases with the etchingduration, whereas the crack base is deepened at the same etching rate asthe planar surface, so that the crack depth remains unchanged in a firstapproximation. Therefore, with an increasing etching duration the crackprofile assumes a spherical shape.

The photograph of FIG. 2 shows the surface of a ground and, as has beendescribed further above for the quartz glass tube, honed quartz glasssample. Despite the fine-grained polishing agent, grinding marks can bedetected on the surface. However, apart from the grinding marks, thesurface shown in FIG. 2 appears to be smooth; the mean surface roughnessR_(a) amounts to about 0.1 μm.

The definition of the surface roughness R_(a) follows from EN ISO 4287and the measurement conditions from EN ISO 4288 or EN ISO 3274,depending on whether the surface of the measurement sample has beenfinished, like in the instant case, by grinding and honing (non-periodicsurface profile) or by turning (periodic surface profile).

The result of a 1-minute etching of the surface treated in this way in a68% HF solution is shown in FIG. 3. In this photograph it is stillpossible to make out the grinding marks as weak lines, the etchingtreatment also making visible grinding marks that have not been visibleor have only been slightly visible in the preceding photograph. Inaddition, and in a particularly conspicuous manner, cracks have now alsobecome visible that cannot be seen in the photograph of the unetchedsurface. The surface is covered with cracks which are narrowlydistributed and occur independently of the course of the grinding marks.After the etching treatment the cracks have a width of 7 μm. It has beendetected in a separate etching test that the crack depth is below 1 mm.The mean surface roughness R_(a) is about 0.5 μm in the etched surface,measured by means of a surface roughness measurement device. Cracks withsuch a depth and width in the inner wall of a quartz glass cylinder canstill be closed during collapsing onto the core rod in an RIC methodwithout special measures being needed with respect to a particularly lowviscosity of the inner wall.

A distinct deterioration of the surface quality, however, will beobserved when the etching duration is prolonged to 50 minutes, as shownin the surface photograph of FIG. 4. The mean crack width of the cracksis now 140 μm after this etching treatment. The crack depth can bedetermined in a simple way by means of a standard surface roughnessmeasurement device.

To determine the impact which a special kind of treatment of the innerwall of a quartz glass cylinder has on the quality of the boundarysurface obtained in an RIC method between the cylinder and a core rodinserted thereinto, quartz glass cylinders are produced with adifferently treated inner bore (see Table 1) and used in an RIC method,which will be described in more detail in the following.

A core rod is inserted into and fixed in the hollow cylinder of quartzglass having a quality of the inner bore as indicated in Table 1. Eachof the core rods are produced by means of MCVD methods by depositingSiO₂ cladding and core glass layers on the inner wall of a substratetube. To obtain core glass rods having a particularly low OH content (<1wt ppb), hydrogen-free start substances are used, the deposition zonebeing heated by means of an electrically heated annular furnacesurrounding the substrate tube, which is moved in the direction of thelongitudinal axis of the substrate tube.

In all of the tests the cylinder has an outer diameter of 150 mm and aninner diameter of 60 mm, and the diameter of the core rod is 58 mm eachtime.

The composite of hollow cylinder and core rod is supplied in verticalorientation from above to an electrically heated furnace at apredetermined feed rate and is heated therein zonewise to a temperatureranging from 2000° C. to 2400° C., a preform being drawn from thesoftened area. The advance movement is the same in all cases and thedraw-off rate is controlled such that the desired diameter of thepreform of 85.0 mm +/−0.5 mm is obtained. The other process parameters,of which the drawing temperature must particularly be named, are notchanged. A vacuum ranging from 2 kPa to 10 kPa is maintained in theannular gap between core rod and hollow cylinder of 1 mm.

The quality of the boundary surface between the core region of thepreform and the cladding glass provided by the hollow cylinder isexamined by microscopy, special attention being paid to elongatedbubbles along the boundary surface. Moreover, the fiber strength of thefibers obtained from the preforms is measured by stretching said fibersby 1% of their initial length, and the costs entailed by the productionof the quartz glass cylinder are estimated. The qualitative resultsobtained are listed in the last three columns of Table 1, the symbol“++” meaning “very good”, “+” “good” and “−” poor.

The tensile strength of the fibers as indicated in column 5 of the tabledescribes the purity of the boundary surface. Particles in the area ofthe boundary surface between the quartz glass deriving from the core rodand the cladding glass may impair the fiber strength. Purity can beimproved at any rate by etching the cylinder prior to the RIC process.In tests according to Table 1, an etching removal in HF-containingsolution of about 1 μm/min was set. However, the quality of the surfaceas a consequence of the preceding mechanical treatment must be takeninto account in the etching process. The etching removal in the area asindicated in column 3 of Table 1 leads to a crack width that is twice aslarge in terms of figures. When the existing subsurface cracks areenlarged by etching to such an extent that crack widths of more than 100μm are created, this will be noticed in a deterioration of the boundarysurface quality. This is shown by tests 10 and 11 in which due to anetching removal of 100 μm a crack width of 200 μm has been produced, ascompared with the better boundary surface qualities obtained in tests 4and 5 at about half the removal depth. Although subsurface cracks can beminimized by a particularly troublesome mechanical treatment, theefforts required therefor cannot be justified economically, as hinted atby samples no. 9 and no. 12.

TABLE 1 Treatment of the inner wall of the hollow cylinder Removal byQuality of Tensile Max. crack depth etching treatment the strength bymechanical (=½ × crack boundary of No. treatment [mm] width) [um]surface fiber Costs 1 3 0 + − ++ 2 2 0 + − ++ 3 1 0 + − + 4 2 40 + + ++5 2 30 ++ + ++ 6 2 20 ++ + ++ 7 1 40 ++ + + 8 1 20 ++ + + 9 0.025 20 ++++ − 10 1 100 − + + 11 2 100 − + + 12 0.025 100 ++ ++ −Hence, it follows from the data of Table 1 that disadvantageous resultsare obtained in both a coarse and a very fine mechanical treatment ofthe inner bore of the hollow cylinder, but also in the absence of anetching process or in the case of a long etching process.

1. A method for producing a quartz glass article, for producing anoptical component, the method comprising the steps of: providing aquartz glass cylinder having an inner bore therein, mechanicallytreating the inner bore to a final dimension; and then applying anetching treatment to the inner bore, wherein the step of mechanicallytreating the inner bore comprises a plurality of removal processes eachwith a successively smaller removal depth such that, the inner bore hasclosed subsurface cracks therein, and all of the closed subsurfacecracks in the inner bore have depths of not more than 2 mm after thelast removal process, and wherein the inner bore is subsequentlysubjected to the etching treatment so as to produce an etching removalwith a depth of not more than 50 μm, and such that the inner bore has anetched structure that has cracks therein, all of said cracks having adepth of not more than 2.0 mm and a width of not more than 100 μm. 2.The method according to claim 1, wherein the etching treatment yields anetching removal with a depth of not more than 25 μm.
 3. The methodaccording to claim 1, wherein the etching treatment yields an etchingremoval with a depth of not more than 10 μm.
 4. The method according toclaim 1, wherein the etching treatment yields an etching removal with adepth of at least 2.5 μm.
 5. The method according to claim 1, whereinthe etching treatment includes a first etching step in a first etchingsolution containing hydrofluoric acid, and a second etching step in asecond etching solution containing nitric acid.
 6. The method accordingto claim 1, wherein the etching treatment is carried out at a meanetching rate of not more than 3 μm/min.
 7. The method according to claim6, wherein the mean etching rate is not more than 1 μm/min.
 8. Themethod according to claim 6, wherein the mean etching rate is not morethan 0.1 μm/min.
 9. The method according to claim 1, and furthercomprising inserting a core rod in the inner bore of the quartz glasscylinder, and forming a preform from said core rod and said quartz glasscylinder.